10 impossibilities conquered by science

What is truly impossible? To accompany Michio Kaku's article on the physics of impossibility, we have rounded up 10 things that were once thought scientifically impossible. Some were disproved centuries ago but others have only recently begun to enter the realm of possibility.

1. Analysing stars

In his 1842 book The Positive Philosophy, the French philosopher Auguste Comte wrote of the stars: "We can never learn their internal constitution, nor, in regard to some of them, how heat is absorbed by their atmosphere." In a similar vein, he said of the planets: "We can never know anything of their chemical or mineralogical structure; and, much less, that of organized beings living on their surface."

Comte's argument was that the stars and planets are so far away as to be beyond the limits of everything but our sense of sight and geometry. He reasoned that, while we could work out their distance, their motion and their mass, nothing more could realistically be discerned. There was certainly no way to chemically analyse them.

Ironically, the discovery that would prove Comte wrong had already been made. In the early 19th century, William Hyde Wollaston and Joseph von Fraunhofer independently discovered that the spectrum of the Sun contained a great many dark lines.

2. Meteorites come from space

Astronomers look away now. Throughout the Renaissance and the early development of modern science, astronomers refused to accept the existence of meteorites. The idea that stones could fall from space was regarded as superstitious and possibly heretical - surely God would not have created such an untidy universe?

The French Academy of Sciences famously stated that "rocks don't fall from the sky". Reports of fireballs and stones crashing to the ground were dismissed as hearsay and folklore, and the stones were sometimes explained away as "thunderstones" - the result of lightning strikes.

It was not until 1794 that Ernst Chladni, a physicist known mostly for his work on vibration and acoustics, published a book in which he argued that meteorites came from outer space. Chladni's work was driven by a "fall of stones" in 1790 at Barbotan, France, witnessed by three hundred people.

Chladni's book, On the Origin of the Pallas Iron and Others Similar to it, and on Some Associated Natural Phenomena, earned him a great deal of ridicule at the time. He was only vindicated in 1803, when Jean-Baptiste Biot analysed another fall of stones at L'Aigle in France, and found conclusive evidence that they had fallen from the sky.

3. Heavier-than-air flight

The number of scientists and engineers who confidently stated that heavier-than-air flight was impossible in the run-up to the Wright brothers' flight is too large to count. Lord Kelvin is probably the best-known. In 1895 he stated that "heavier-than-air flying machines are impossible", only to be proved definitively wrong just eight years later.

Even when Kelvin made his infamous statement, scientists and engineers were closing rapidly on the goal of heavier-than-air flight. People had been flying in balloons since the late eighteenth century, and by the late 1800s these were controllable. Several designs, such as Félix du Temple's Monoplane, had also taken to the skies, if only briefly. So why the scepticism about heavier-than-air flight?

The problem was set out in 1716 by the scientist and theologian Emanuel Swedenborg in an article describing a design for a flying machine. Swedenborg wrote: "It seems easier to talk of such a machine than to put it into actuality, for it requires greater force and less weight than exists in a human body."

Swedenborg's design, like so many, was based on a flapping-wing mechanism. Two things had to happen before heavier-than-air flight became possible. First, flapping wings had to be ditched and replaced by a gliding mechanism. And secondly, engineers had to be able to call on a better power supply - the internal combustion engine. Ironically, Nicolaus Otto had already patented this in 1877.

4. Space flight

From atmospheric flight, to space flight. The idea that we might one day send any object into space, let alone put men into orbit, was long regarded as preposterous.

The scepticism was well-founded, since the correct technologies were simply not available. To travel in space, a craft must reach escape velocity - for vehicles leaving Earth, this is 11.2 kilometres per second. To put this figure into perspective, the sound barrier is a mere 1,238 kilometres per hour, yet it was only broken in 1947.

Jules Verne proposed a giant cannon in his novel From the Earth to the Moon. However, such a sudden burst of acceleration would inevitably kill any passengers instantly, and calculations have shown no cannon could be powerful enough to achieve escape velocity.

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